The present invention relates to airfoils and other aerodynamic and hydrodynamic surface structures which generate tip vortices due to motion relative to a fluid such as air or water, more particularly aircraft wings which generate wing tip vortices.
Tip vortex generation is a well-known phenomenon involving surface-fluid interaction which is manifest in various aerodynamic and hydrodynamic contexts. It is of particular concern to aircraft designers, who for many years have been considering various approaches to alleviating the attendant undesirable effects of wing tip vortex generation.
It is generally understood that the air flowing over the aircraft wing achieves a higher velocity and a correspondingly lower pressure than the air flowing under the wing, resulting in an upward force called lift; however, due to this pressure differential, the air near each wing tip tends to flow from beneath the wing to above the wing, this "leakage" around the tips thus forming wing tip vortices. One suggested explanation is that the air flowing over the wing tends to flow slightly inwards from the wing tips; the air flowing under the wing tends to flow slightly outwards toward the wing tips. When these airflows meet at the trailing edge numerous little vortices are formed, which unite to form two large vortices, one at each wing tip.
Whatever the precise explanation, these wing tip vortices represent a real phenomenon with real, potentially negative effects. An induced downward velocity behind the wing lowers the effective angle of attack, which in turn causes reduced lift and induced drag, a component of the overall drag. The reduced lift can also be viewed as attributable to decrease in average pressure below the wing. Lower effective angle of attack, reduced lift and induced drag are especially critical factors during takeoff, ascent, cruise, descent, and landing. Moreover, wing tip vortices which are trailing the moving wing and remain for a period of time constitute a hazardous condition for aircraft which follow in its wake. This wake turbulance is of especially alarming magnitude for very large aircraft. Air traffic control personnel at airports must be ever cognizant of wake turbulance in timing and spacing incoming and outgoing flights.
Wing tip vortex generation and concommitant deleterious effects can be mitigated and wing efficiency improved by increasing the aspect ratio of the wing, which is the square of the wing span divided by the wing area. One way to accomplish this is to simply increase the aspect ratio by dimensionally increasing the wing span and/or decreasing the wing chord. Another way, and one which has been the subject of many patents, is to attempt to effectively increase the wing span and thus effectively increase the aspect ratio by reducing or mitigating the wing tip vortex generation itself.
Various approaches have been taken in attempting to attenuate the wing tip vortices. Generally speaking, they may be viewed as falling into two categories. The first utilizes external structural shape or configuration for directing or redirecting natural airflow so as to somehow counteract vortex generation. The second, which may have configurational aspects, directs airflow or gas flow from a source originating from within the aircraft for this purpose. The source of the fluid discharge can be, for example, a plenum, blower, compressor or engine.
An example of the former category is provided by Hackett, U.S. Pat. No. 4,190,219, which discloses a boom attached to a vertical vane located adjacent to and aft the wing tip. This device acts as a vortex diffuser for attenuating the vortex once it has formed and is trailing the wing tip. Frakes, U.S. Pat. No. 4,478,380, an example which takes a different approach, discloses a scoop located near the wing tip. The inlet of the scoop is on the lower surface of the wing near the leading edge; the outlet is on the upper surface near the trailing edge. High pressure air from the lower surface is fed to the upper surface outlet with the result of reducing the pressure differential at the wing tip trailing edge which in turn results in reduction of the wing tip vortex. Finch, U.S. Pat. No. 4,108,403, discloses a wing tip construction which extends outward and aft and curves downward Nelson, U.S. Pat. No. 4,017,041, discloses retractable foils which extend upwardly or downwardly from a location at or near the wing tip.
Configurational approaches to tip vortex reduction have met with varying degrees of qualified success. One reason is that structural deviation or amplification which is designed to reduce the tip vortex also inherently changes the aerodynamics of the aircraft, with the result that the concomitant aerodynamic penalties, e.g., reduced lift or increased drag, will at least somewhat offset the aerodynamic benefits of the tip vortex reduction. The resultant change in fluid flow pattern will similarly alter the aerodynamic balance. Some approaches are multipurpose and the vortex-related purpose is secondary or ancillary. Moreover, these approaches seek to limit the tip vortex once it has been more or less generated. The effectiveness of tip vortex attenuation tends to increase as it attacks the tip vortex at an increasingly early stage in its formation.
Conventional fluid discharge approaches share these deficiencies. They inject fluid into the aerodynamic scheme, and incorporation therein of configurational aspects will even further disturb the aerodynamic balance. In some cases nonexclusivity of the vortex attenuation purpose is a compromising factor. Temporal lateness of the fluid-vortex interaction in the vortex formation process also militates against accomplishing objectives of vortex attenuation.
Freed, U.S. Pat. No. 3,841,587, and Lessen, U.S. Pat. No. 3,881,669, disclose discharging fluid rearwardly and generally perpendicular to the wingspan from the trailing edge at or near the wing tip. Lessen incorporates a counterrotational aspect into the fluid stream vis-a-vis the vortex. Both Freed and Lessen seek to act on the already-established trailing vortex. Wake turbulance is alleviated but the afore-discussed aerodynamic goals of improving lift and reducing drag are largely ignored.
A capture device at the wing tip which is concave downward and inward is disclosed by Boppe et al, U.S. Pat. No. 4,382,569. The device seeks to capture lower-to-upper wing tip leakage and utilize negatively pressured fluid discharge to aspirate this crossflow and release it somewhere else. Although Boppe does seek to attack the source of the vortex generation, the device configuration and aspirated fluid release serve to negative the positive aerodynamic effects.
Cornish III, U.S. Pat. No. 3,480,234, discloses releasing spanwise fluid flow from a point some distance from the wing tip and directing it over the upper wing surface. This discharged spanwise fluid flow along the upper wing causes its own locked vortex to form from the combination thereof with entrained air which is carried along with it. The vortex is coaxial with the spanwise flow and is rotating in a direction which brings the main chordwise airflow over the locked vortex and downward to be reattached to the wing's upper surface behind the vortex. This limits flow separation of airflow across the upper surface of the wing, which typically happens at relatively low speeds, e.g., takeoff or landing, or when the wing is at a high angle of attack. This spanwise fluid dynamic also pushes the already-formed wing tip vortices outward, thus increasing effective wing span and decreasing induced drag. Although configurational aerodynamic penalties are minimized, vortex attenuation is limited in effectiveness and of secondary consequence in view of the fact that the desired fluid interaction is directed toward the primary purpose of flow separation reduction; the spanwise fluid interaction with an established vortex is thus seen as less than ideal for accomplishing the vortex attenuation purpose.
In an embodiment disclosed by Griswold II, U.S. Pat. No. 4,477,042, at FIG. 11 therein, the wing tip is contoured downward and the fluid is discharged therefrom linearly and tangentially with respect to the upper and lower surfaces near the release slot. The downward curvature and fluid discharge together act on the source of the vortex generation so that a milder vortex is shed outboard the wing tip. Again, a configurational aspect negatively impacts the aerodynamics; in this case, structural alteration at the wing tip works to the detriment of wing efficiency. The same can be said for a jet stream which is discharged outwardly at a downward angle from the wing tip. Moreover, although this fluid discharge methodology attacks the tip vortex at an early stage in its formation, it nevertheless allows its formation to a point and then attenuates it.
Yuan, U.S. Pat. No. 3,692,259, discloses both strictly configurational and internally-derived fluid discharge approaches. In a configurational embodiment a rotating blower at the wing tip draws air from the lower wing and releases it through a slot at the wing tip. Other embodiments of Yuan at '259 seek to avoid configurationally-derived aerodynamic detriment by deriving the discharged fluid internally and utilizing an ordinary wing tip; in other words, rather than angle the internal fluid conduit near the release point in accordance with the contour of a downwardly curved wing tip, e.g., as in Griswold II, the internal fluid conduit is angled downward near the release point but the wing tip remains relatively horizontal. In theory Yuan at '259 is adaptable to any wing tip, and the wing tip is permitted to retain its original design integrity; in practice, however, some difficulties may be posed in internally fitting the fluid conduit in the desired manner and for effectuating the desired release in view of the panoply of wing thicknesses and shapes. Yuan at U.S. Pat. No. 3,936,013 addresses this by externally configuring a tube projecting spanwise from the wing tip, the tube discharging fluid in selected directions, but the tube itself exacts an aerodynamic price.
Yuan at '259 perhaps implicitly recognizes that it is increasingly effective to attack the vortex with increasing proximity to the source of the vortex. Yuan's methodology is to set up a fluid flow which is counter-directional to the crossflow from the lower wing to the upper wing. Although Yuan at '259 seeks to provide the counter-rotational flow to this end, its effectiveness is limited because the release point of the fluid is located at or near the outboard-most point of the wing tip, midway or nearly midway between the upper surface and the lower surface, and the fluid flow is directionally more outward and spanwise and less downward and counter-rotational; moreover, an approach which attempts to reduce the vortex by meeting it head-on as it crossflows from the lower to the upper wing is problematical. Another embodiment therein seeks to improve the rotational flow by adding a second fluid jet. Yuan at '259 also discloses an embodiment utilizing a rotating cylinder at the wing tip which simultaneously discharges and rotates the released fluid, seeking to improve the rotational aspect of the flow; still, the requisite machinery itself carries aerodynamic and practical disadvantages.
It is also desirable to have the flexibility to vary the fluid interaction with the tip vortex, in order to accomplish the vortex-attenuation purpose. It is a relatively simple matter to mechanically regulate the fluid flow in terms of mass flow; nevertheless, conventional approaches to vortex attenuation require more complex variability of the fluid flow in terms of intensity and direction, which is less easily accomplished, in order to meet varying aerodynamic demands. Yuan at '259, for example, proposes to vary the chordwise flow in accordance with varying slot widths or hole diameters; of course, utilization of apertures of varying sizes and shapes in this manner is limited because these sizes and shapes cannot be changed. For the above-described rotating wing tip, Yuan varies the rotational speed of the cylinder.
In any case, a review of conventional approaches to vortex attenuation points up the desirability of a simple, efficient and adaptable approach thereto which effectively obstructs or impedes crossflow from the lower wing surface to the upper wing surface, and which accommodates a variety of airfoil shapes and aerodynamic situations.
The present invention utilizes a well-known phenomenon of fluid motion, viz., the tendency of a fluid jet to adhere to an adjacent curved surface. This attraction/entrainment phenomenon is known as the Coanda effect, named after the inventor for a French Patent which issued in 1932. For an excellent discussion on the Coanda effect, see Wille, R., and Fernholz, H., "Report on the first European Mechanics Colloquium, on the Coanda effect," Journal of Fluid Mechanics, vol. 23, part 4, pp. 801-819, Cambridge at the University Press (1965).
Utilization of the Coanda effect has been known in various aerodynamic contexts, notably in circulation control applications for aircraft such as STOL and X-wing. Examples are found at Cycon et al., U.S. Pat. No. 4,770,607; Bennett et al., U.S. Pat. No. 4,736,913; Thomas, U.S. Pat. No. 4,682,746; Moore, U.S. Pat. No. 4,674,716; Groth, U.S. Pat. No. 4,146,197; Hirt et al., U.S. Pat. No. 4,019,696.